1. Field of the Invention
The disclosed technology relates to a method of manufacturing a light emitting diode provided with a photonic crystal structure and also relates to a light emitting diode manufactured therewith.
2. Description of the Related Technology
Light emitting diodes (LEDs) based on III-nitride type materials (GaN LEDs) are a heavy research subject. The type III-nitride materials is particularly gallium nitride (GaN) and higher alloys thereof, including aluminium gallium nitride (AlGaN), indium gallium nitride (InGaN), indium aluminium gallium nitride (1 nAlGaN) and the like. These materials may be used as blue light emitting diodes. The generation of light in semiconductor materials with first and second active layers is known per se: upon application of a certain voltage, charge carriers in the active layers recombine to generate photons. However, generated photons need to be able to leave the material. The issue is that radiation enters waveguided modes internal to the semiconductor rather than radiation modes. Light generated inside the semiconductor bounces around due to total internal reflection, and there is a high probability that the light will be absorbed before it can escape from the semiconductor material. One ways of reducing the issue is based on application of antireflection coatings and a textured surface.
An alternative way of reducing the issue is the provision of photonic crystals, or more adequately, a photonic crystal structure. Such a photonic crystal structure is typically designed as a dielectric layer or stack of dielectric layers provided with periodically distributed holes or trenches. The periodical distribution is for instance a triangular array. Electromagnetic waves, such as light, can be confined therewith in a small volume of three dimensions. Hence, the photonic crystals defining such small volumes are able to improve emission and guide light. As the external quantum efficiency of GaN LEDs is often in the vicinity of 10 percent, the formation of photonic crystals can improve the efficiency of a GaN LED in a substantial manner. Here the external quantum efficiency is the product of the internal quantum efficiency and the extraction efficiency. This improvement of efficiency is deemed due to a strongly modified emission pattern due to the scattering of waveguided modes out of the semiconductor material.
The periodically distributed trenches of the photonic crystal structure are typically defined by dry etching. This manufacturing process is complicated by the difficulty of etching the material, which is extremely hard and chemically inert. It inevitably results in imperfections leading to out-of-plane optical losses.
U.S. Pat. No. 7,642,108 discloses an improved method for manufacturing such GaN LEDs with a photonic crystal structure. This method uses a substrate transfer process in combination with epitaxial lateral overgrowth (ELOG). As known per se, GaN has the ability to grow in lateral directions when growth starts from a patterned template. The method of U.S. Pat. No. 7,642,108 provides a pattern to a substrate surface in the form of a mask. The mask is overgrown with ELOG. It is observed herein, that the lateral overgrowth leads to layer growth with reduced defect density. Hence, at the location of the mask GaN or AlGaN material is formed with a comparatively low defect density. At other locations, i.e. from where the growth started, the semiconductor material is formed with a comparatively high defect density. Thereafter an active layer structure is grown This active layer structure comprises a first active layer, that is n-type doped, and a second active layer, that is p-type doped. After this growth and the provision of a top electrode layer and protective layers as known per se to the skilled person, a substrate transfer process is carried out. Herein, a carrier is applied at the top side, and the original substrate is removed, for instance by grinding and etching. Therewith, the layer grown by the ELOG is exposed. The method of U.S. Pat. No. 7,642,108 now proceeds with etching, wherein the areas with a high defect density are removed. This etching continues through the active layer structure, in accordance with the same pattern. Probably, this is desired, as the quality of the active layers grown on the areas with the comparatively high defect density may well be lower than the quality of the active layers grown on the areas with the comparatively low defect density. The etching may be continued up to the carrier, but that is not necessary.
It is a disadvantage of the method known from U.S. Pat. No. 7,642,108 that it is still necessary to etch through the layers of III-nitride material. As stated before, the etching is difficult and requires a heavy etchant, such as a chlorine-based plasma. It may thus well damage the layers of III-nitride material. As not merely the layers formed by ELOG, but also the active layer structure is etched, this may have a negative impact on the reliability of the resulting device.